The lawsuit did not specifically name Cage as the owner, but Cage's publicist confirmed that the actor bought the skull in March 2007 from a Beverly Hills gallery, I.M. Chait.

The "National Treasure" actor is not accused of wrongdoing, and authorities said he voluntarily agreed to turn over the skull after learning of the circumstances.

Alex Schack, a publicist for Cage, said in an email that the actor received a certificate of authenticity from the gallery and was first contacted by U.S. authorities in July 2014, when the Department of Homeland Security informed him that the skull might have been stolen.

Following a determination by investigators that the skull in fact had been taken illegally from Mongolia, Cage agreed to hand it over, Schack said.

The I.M. Chait gallery had previously purchased and sold an illegally smuggled dinosaur skeleton from convicted paleontologist Eric Prokopi, whom Bharara called a "one-man black market in prehistoric fossils."

The Chait gallery has not been accused of wrongdoing. A representative did not return a request for comment on Monday.

It was unclear whether the Nicolas Cage skull was specifically connected to Prokopi, who pleaded guilty in December 2012 to smuggling a Tyrannosaurus bataar skeleton out of Mongolia's Gobi desert and was later sentenced to three months in prison.

As part of his guilty plea, Prokopi helped prosecutors recover at least 17 other fossils.

Assistant U.S. Attorney Martin Bell, who prosecuted Prokopi, was also the lead government lawyer in the Cage case, according to court records.

The Tyrannosaurus bataar, like its more famous relative Tyrannosaurus rex, was a carnivore that lived approximately 70 million years ago. Its remains have been discovered only in Mongolia, which criminalized the export of dinosaur fossils in 1924.

Since 2012, Bharara's office has recovered more than a dozen Mongolian fossils, including three full Tyrannosaurus bataar skeletons.

"Each of these fossils represents a culturally and scientifically important artifact looted from its rightful owner," Bharara said last week.